The present invention relates to a cutting insert for chipbreaking machining tools, in particular for milling tools such as facemilling tools, and to a process for making the insert.
Such an insert is typically manufactured by form-pressing and sintering of an insert-forming powder material to form a body which comprises a top chip surface, a suitably planar bottom surface that can rest upon a seat surface of the machining tool, and at least one edge surface extending between the top and bottom surfaces. The edge surface, which can be placed in abutment with at least one cooperating side abutment surface of the tool, is generally inclined at an acute angle with respect to the chip surface and at an obtuse angle with respect to the bottom surface, whereby a cutting edge is formed along the intersection of the chip surface and the edge surface, adjacent to which cutting edge there are one or several relief or clearance faces.
For the manufacturing of such cutting inserts, in particular indexable cutting inserts, of hard metal. a direct-pressing method is frequently used, in which a hard metal-forming powder first is formed to the intended shape in a suitable pressing die and then given the final strength and size by sintering in an oven at a temperature above 1000.degree. C. The pressing operation as such has been further developed over the years and is today so advanced that it enables the formation of the cutting edges and adjacent chipforming faces and possible reinforcing faces with great dimensional precision. However, during the sintering operation a shrinkage takes place (usually amounting to about 18% of the original length in each dimension) and due to this, the cutting insert loses some of its original precision.
For some types of machining, e.g., some sorts of facemilling, the requirements of form and dimensional precision have become more rigorous over the last years. Particularly insert geometries with a positive cutting edge require a very high degree of dimensional accuracy to guarantee a satisfactory function at small tooth feeds. These precision requirements have up to now been met by so-called contour grinding, which consists of after-grinding the surface(s) adjacent to the individual cutting edge in one step after sintering. However, a serious disadvantage of such contour grinding is that it causes modifications in the micro-geometry of the insert, i.e., in the surface structure of the insert's edge-shaping parts after a surface treatment such as blasting, face grinding or deposition of a hardness-improving surface layer, which is usually done as soon as possible after the sintering has been finished. Thus, the width of existing negative reinforcing surfaces is altered, as well as the distance from the cutting edge to the chipforming surfaces. In practice, this means that the chipforming ability and the cutting performance of the cutting insert are diminished and that its strength and life are reduced.